A dual-responsive RhB-doped MOF probe for simultaneous recognition of Cu2+ and Fe3+

Based on the dual response of RhB@UiO-67 (1:6) to Cu2+ and Fe3+, a proportional fluorescent probe with (I392/I581) as the output signal was developed to recognize Cu2+ and Fe3+. Developing highly sensitive and selective trace metal ions probes is crucial to human health and ecological sustainability. In this work, a series of ratio fluorescent probes (RhB@UiO-67) were successfully synthesized using a one-pot method to enable fluorescence sensing of Cu2+ and Fe3+ at low concentrations. The proportional fluorescent probe RhB@UiO-67 (1:6) exhibited simultaneous quenching of Cu2+ and Fe3+, which was found to be of interest. Furthermore, the limits of detection (LODs) for Cu2+ and Fe3+ were determined to be 2.76 μM and 0.76 μM, respectively, for RhB@UiO-67 (1:6). These values were significantly superior to those reported for previous sensors, indicating the probe’s effectiveness in detecting Cu2+ and Fe3+ in an ethanol medium. Additionally, RhB@UiO-67 (1:6) demonstrated exceptional immunity and reproducibility towards Cu2+ and Fe3+. The observed fluorescence quenching of Cu2+ and Fe3+ was primarily attributed to the mechanisms of fluorescence resonance energy transfer (FRET), photoinduced electron transfer (PET), and competitive absorption (CA). This work establishes a valuable foundation for the future study and utilization of Cu2+ and Fe3+ sensing technologies.

www.nature.com/scientificreports/change in single-emission fluorescence intensity for sensing, which was often influenced by uncontrollable variables like photobleaching, light scattering, and concentration inhomogeneity 21 .Ratiometric fluorescent probes could effectively overcome these drawbacks due to their multi-occurrence self-calibration signals.Considering the excellent resultant stability and unique optical properties of Zr-MOF, combining Zr-MOF with fluorescent dyes is regarded as an intuitive and effective method, which not only restricts the migration and aggregation of fluorescent dye molecules by Zr-MOF but also generates peculiar fluorescent properties through the subjectobject interaction 22 .
Based on the above factors, a series of ratiometric fluorescent probes were successfully prepared in this work by encapsulating rhodamine B (RhB) into UiO-67.In addition, Cu 2+ and Fe 3+ , as hard acids, can easily coordinate with hard bases such as carboxyl groups 23,24 .The organic ligands biphenyl-4,4′-dicarboxylate (H 2 BPDC) and RhB have abundant carboxyl groups, which can easily coordinate with each other and Cu 2+ and Fe 3+ , significantly improving the selectivity and detection limits of RhB@UiO-67 for Cu 2+ and Fe 3+25 .The LODs of Cu 2+ and Fe 3+ for RhB@UiO-67 were 2.76 μM and 0.76 μM, respectively.Additionally, the probe was also shown to have excellent potential for sensing Cu 2+ and Fe 3+ in terms of its sensitivity, selectivity, immunity to interference as well as reproducible performance.Furthermore, possible sensing mechanisms have been systematically investigated.Consequently, this current study provides a straightforward and effective strategy for exploring the efficient sensing of trace metal ions (Cu 2+ and Fe 3+ ).

Experimental Preparation of LMOFs
UiO-67 was prepared with adjustments based on previous literature 26 .ZrCl 4 (0.5 mmoL, 116.52 mg), and H 2 BPDC (0.5 mmoL, 121.11 mg) were dissolved in 30 mL of DMF and subjected to sonication for 30 min to dissolve.Then, the resulting solution was transferred to a Teflon jar and heated at 120 ℃ for 24 h.The strip samples were subsequently cooled to room temperature and washed three times each with DMF and methanol.The white powder obtained was dried at 120 °C for 48 h, followed by further characterizations.Within this experimental procedure, a series of RhB@UiO-67 were synthesized with varying RhB/H 2 BPDC ratios of 1:2, 1:4, 1:6, 1:8, and 1:10, respectively.Additionally, the pore structure characteristics of these materials were assessed to ensure maximum surface area and pore volume.

Fluorescence sensing experiments
The synthesized fluorescent probes were activated under vacuum at 120 °C for 12 h for fluorescence sensing.The fluorescence properties of UiO-67 and RhB@UiO-67 (1:6) in different solvents were investigated using a spectrophotometer (Hitachi F-7100) with excitation/emission slits of 5.0 nm and photomultiplier voltage of 300 V.The concentration of MOFs was 0.33 mg/mL in selectivity, titration, and immunity experiments.

Density functional theory (DFT) calculation
In this experiment, geometry optimization, the energy levels of lowest unoccupied molecular orbitals (LUMOs) and highest occupied molecular orbitals (HOMOs) were evaluated using density functional theory (DFT) and optimized using Dmol3 of the Materials Studio 2019 package.The all-electron interaction theory (AER) potential accounts for the interaction between electrons and ions.All atoms were allowed to spin unrestricted during the structure optimization process.The GGA-PBE functional and DNP4.4 basis sets were used for the calculations.In the current study, the convergence criterion of the electronic self-consistent field (SCF) loop was set to be 10 -6 with energy 10 -5 Ha, force constant 0.002 Ha/Å, displacement 0.005 Å, and value of smearing 0.05 Ha 27,28 .Considering that the metal ions might interact with the carboxyl groups in the organic ligands, H 2 BPDC and RhB, to simplify the computational process, the optimized structure considered only a single carboxyl group and a metal ion on the ligand.All calculations were performed using a solvent model (ethanol) to obtain more accurate and reliable results.

Analysis of real samples
First of all, 100 mg of Alisma plantago-aquatica L. was soaked in 50 mL of ethanol for 24 h and then filtered using a 0.22 μm filter membrane.Eventually, the samples were analyzed with the addition of different concentrations of Cu 2+ and Fe 3+ solutions (1 μM, 5 μM, and 10 μM) for three times.

Characterization of RhB@UiO-67
The N 2 adsorption-desorption isotherms of obtained RhB@UiO-67 materials at 77 K were presented in Fig. 1a.Obviously, the adsorption capacity of N 2 exhibits a rapid increase at lower relative pressures (p/p 0 ), conforming to a typical type I isotherm, indicating the presence of numerous micropores in RhB@UiO-67 29 .Additionally, the NLDFT pore size distributions of RhB@UiO-67 materials were more uniform compared with those of UiO-67.And, in RhB@UiO-67, the BET surface areas were relatively small, possibly because RhB plugged the pore channels of UiO-67 (Fig. 1b).Meanwhile, the BET surface area increased followed by a decrease with an increase of RhB, reaching a maximum at RhB/H 2 BPDC of 1:6 (Table 1).This increase may be caused by the presence of new pore structure shapes, and the decrease may be caused by the continuous addition of RhB to disrupt the symmetry of the original UiO-67 leading to partial structural collapse and blockage of the pore channels 22,23 .RhB@UiO-67 (1:6) was chosen as a potential probe material for metal ion detection due to its significant specific surface area and pore volume.
The PXRD patterns of RhB@UiO-67 materials were very similar to that of pure UiO-67, indicating the wellmaintained structure of UiO-67 after the introduction of RhB (Fig. S2).Therefore, it was likely that RhB was encapsulated within the voids rather than physically adsorbed on the surface of UiO-67.In addition, Fig. 1d demonstrated that the FTIR spectra of UiO-67 and RhB@UiO-67 were highly similar, confirming the encapsulation of RhB and the negligible effect of host-guest encapsulation on the structural integrity of UiO-67 31 .Furthermore, its exceptional thermal stability was demonstrated by its unchanged state at temperatures up to 450 °C (Fig. S3).(d) FTIR spectra between 4000 and 400 cm −1 of RhB@UiO-67 materials.

Photoluminescence properties
Fluorescence performance of RhB@UiO-67 The solid-state fluorescence properties of the organic ligand (H 2 BPDC), vacuum-activated UiO-67, and RhB@ UiO-67 were examined.The presence of abundant aromatic rings in the π-π conjugated system of the organic ligand resulted in a strong emission at 391 nm upon excitation at 322 nm but the fluorescence intensities of both UiO-67 and RhB@UiO-67 were weaker than that of H 2 BPDC due to the aggregation-induced quenching (Fig. S4a).A new fluorescence peak appeared at 582 nm when RhB was introduced.In addition, the fluorescence intensity of RhB@UiO-67 ligand out showed an initial enhancement followed by a weakening trend after the addition of RhB (Fig. S4b).The initial enhancement could be caused by the interaction between RhB and UiO-67.In contrast, the subsequent decrease resulted from the fluorescence quenching of RhB due to aggregation induction.Additionally, the fluorescence behavior of RhB@UiO-67 (1:6) in various solvents (water, MeOH, EtOH, and DMF) was investigated at room temperature.It was evident that RhB@UiO-67 (1:6) demonstrated the best fluorescence behavior in ethanol (Fig S5a).Furthermore, the investigation of the fluorescence stability of RhB@ UiO-67 (1:6) involved an examination of its relative fluorescence intensity at different time intervals and pH levels.The results demonstrated that the relative fluorescence intensity (I 392 /I 581 ) of RhB@UiO-67 (1:6) remained consistent even after being immersed in ethanol for a duration of one week (Fig. S6c).Additionally, the relative fluorescence intensity of RhB@UiO-67 (1:6) showed minimal variation within the pH range of 3 to 10 (Fig. S6b).Afterward, the I 392 /I 581 ratio of RhB@UiO-67 (1:6) was evaluated at different concentrations.Notably, the I 392 /I 581 ratio initially increased with the concentration of RhB@UiO-67 (1:6), reaching its peak at 0.33 mg/mL (Fig. S5), before subsequently declining.Moreover, it was observed that the I 392 /I 581 ratio remained essentially stationary at 60 s (Fig. S6a).Thus, the optimal concentration and incubation time for RhB@UiO-67 (1:6) were 0.33 mg/ mL and 60 s, respectively.

Fluorescence detection of Cu 2+ and Fe 3+
To investigate the impact of RhB doping on the fluorescence properties of UiO-67, photoluminescence sensing experiments were conducted on a range of metal ions.Figure 2a illustrates that RhB@UiO-67 (1:6) presents remarkable selectivity towards Cu 2+ and Fe 3+ in comparison to other metal ions.The relative fluorescence intensity (I ligand /I RhB ) of RhB@UiO-67 (1:6) exhibited a 64.4% increase for Cu 2+ and an 84.1% decrease for Fe 3+ .However, when examining other potential interfering metal ions, the relative fluorescence intensity (I ligand /I RhB ) of RhB@UiO-67 (1:6) did not change significantly compared to the blank solution (Fig. 2).Additionally, the fluorescence intensity of RhB@UiO-67 (1:6) experienced a decrease across all ligand wavelengths during the detection of selected metal ions (Cu 2+ and Fe 3+ ) in comparison to UiO-67 (Figs. 2a and S7a).Nevertheless, RhB@UiO-67 (1:6) exhibited a noteworthy increase in relative fluorescence intensity (I ligand /I RhB ) compared to the blank solution when detecting Cu 2+ , which may be attributed to the interaction between Cu 2+ and H 2 BPDC/ RhB.In addition, it is evident that the fluorescence intensity of RhB@UiO-67(1:6) for simultaneous Cu 2+ /Fe 3+ sensing is lower than that for Cu 2+ sensing, yet higher than that for Fe 3+ sensing (Fig. S7a).This discrepancy may be attributed to the competition for energy transfer from RhB@UiO-67(1:6) to Cu 2+ or Fe 3+ in the context of simultaneous Cu 2+ /Fe 3+ sensing.
In view of the above, it can be concluded that RhB@UiO-67 (1:6) possesses the potential to effectively monitor the Cu 2+ and Fe 3+ aspects through the utilization of a multi-response sensing model.Interestingly, RhB@ UiO-67 (1:6) exhibited superior sensitivity compared to other probes designed to detect Cu 2+ and Fe 3+ (Table S1).
The probe must possess the capacity to detect the target analyte when other interfering ions are present.Consequently, anti-interference experiments were performed on RhB@UiO-67 (1:6).As shown in Fig. 4a and  b, it is evident that the addition of Cu 2+ and Fe 3+ resulted in the quenching of RhB@UiO-67 (1:6) when coexisting with other metal ions.This finding suggests that RhB@UiO-67 (1:6) is less affected by other interfering ions when sensing Cu 2+ and Fe 3+ .As a result, RhB@UiO-67 (1:6) shows promise for selectively sensing Cu 2+ and Fe 3+ in practical applications.
The ability to regenerate the probe material is also crucial in evaluating its actual sensing performance.To assess the regeneration capability, the probe material was washed with ethanol, followed by centrifugation and drying before conducting subsequent sensing experiments.The results presented in Fig. 4c and d demonstrate that the relative fluorescence intensity of RhB@UiO-67 (1:6) returned to its initial value after several consecutive sensing experiments.Furthermore, the pore structure of RhB@UiO-67 (1:6) remained essentially unchanged following 5 cycles of regeneration in Cu 2+ and Fe 3+ ion solutions, as depicted in Fig. S9a,b and Table S2.However, the BET-specific surface area of RhB@UiO-67 (1:6) began to decrease after 8 cycles, potentially due to the degradation of pore channels by Cu 2+ or Fe 3+ ions after 8 cycles, as shown in Fig. S9c,d and Table S2.These results indicate that the potential reusability of RhB@UiO-67 (1:6) in detecting Cu 2+ and Fe 3+ ions.
Subsequently, FTIR analysis revealed the presence of a peak at 1650 cm -1 , which can be attributed to the carboxylate stretching vibration.There was a blue shift of the peak at 1699 cm -1 owing to the interaction between Cu 2+ /Fe 3+ ions and the carboxyl groups present in the organic ligand H 2 BPDC and RhB (Fig. S10) 35,36 .Furthermore, the XPS spectra revealed the presence of additional peaks, namely the Cu 2p peak at 935.55 eV, when RhB@ UiO-67 (1:6) interacted with Cu 2+ ions, and the Fe 2p peak at 722.13 eV, when RhB@UiO-67 (1:6) interacted with Fe 3+ ions, which were absent in the RhB@UiO-67 (1:6) material (Fig. S11).Moreover, the high-resolution XPS spectrum of the O 1 s orbital exhibited a minimal change in the binding energy of the -OH bond and a www.nature.com/scientificreports/minor variation in the metal-O (metal as Cu/Fe) bond after the interaction with Cu 2+ and Fe 3+ .This observation suggests that Cu 2+ /Fe 3+ ions interact with the carboxyl groups in the organic ligands H 2 BPDC and RhB 37 .
To further substantiate this proposed mechanism, the energies of LUMOs and HOMOs were calculated.The LUMOs of both Cu 2+ -H 2 BPDC and Fe 3+ -H 2 BPDC were found to be lower than that of the ligand, suggesting the occurrence of photoelectron transfer (PET) between RhB@UiO-67 (1:6) and Cu 2+ /Fe 3+ (Fig. 5d) 38 .

Real samples detection
The reliability and adaptability of the method were further evaluated by the determination of Cu 2+ and Fe 3+ in Alisma plantago-aquatica L (one Chinese herbal medicine).The standard addition method was employed, and the results, presented in Table 2, exhibited recoveries ranging from 100.8 to 107.8% with the RSD below 2.73% (n = 3).These findings indicate that the method in the study serves as a robust platform for the detection of Cu 2+ and Fe 3+ .

Conclusion
The ratiometric fluorescent probe RhB@UiO-67 prepared by the one-pot method was used for sensing trace amounts of Cu 2+ and Fe 3+ due to its unique fluorescence properties.Interestingly, RhB@UiO-67 (1:6) exhibited fluorescence behavior with fluorescence quenching for both Cu 2+ and Fe 3+ .Furthermore, the RhB@UiO-67 (1:6) composite demonstrated exceptional immunity, sensitivity, and selectivity towards Cu 2+ and Fe 3+ ions, with LODs of 2.76 μM and 0.76 μM, respectively.These LODs were significantly lower than those reported in previous studies.Additionally, a comprehensive investigation was conducted to elucidate the potential photoluminescence mechanisms, which revealed that the interaction between the carboxyl group in the framework (acting as Lewis base site) and the metal ions Cu 2+ / Fe 3+ Lewis acid sites) may be responsible for competitive absorption (CA), resonance energy transfer (FRET), and photon electron transfer (PET) mechanisms.What's more, combining organic dyes with MOFs with stabilized structures is a very promising method for the recognition of Cu 2+ and Fe 3+ , which paves the way for future investigation and practical applications in the detection of multicomponent metal ions.